Im not so sure that he named himself the Goblin King because of a scooby doo show...
I mean in the movie labyrinth, Jareth, the Goblin King is played by David Bowie, that was out alot earlier.
Dungeons and dragons often mentions the Goblin King long before that, there is even several starter Dungeon Master modules dedicated to that as the main boss.
Did he tell you thats why he chose the name?